1. Field of the Invention
The present invention relates to a motor control device for controlling a motor by estimating the temperature of windings of the motor to prevent burnout of the windings, and a method for calculating an allowable duty cycle time for a machine, which is executed by the motor control device.
2. Description of the Related Art
The winding temperature of a motor increases upon activation of the motor. The winding temperature saturates a certain time after activation of the motor, thereby being maintained at a high temperature (the then obtained temperature will be referred to as a “saturation temperature” hereinafter). When windings having such a saturation temperature continue to be supplied with a current in excess of their heat-resistant temperature, the windings of the motor may be burned out. Even when the temperature of the windings has not saturated, an exceedance of their heat-resistant temperature may burn out the windings of the motor. Therefore, in driving control of the motor, the temperature of the windings is monitored by a temperature sensor. When the monitored winding temperature exceeds a preset temperature, current supply to the windings is stopped to prevent burnout of the windings.
The winding temperature is estimated based on a temperature rise estimation arithmetic expression incorporated in the software of a motor without providing a temperature sensor to the windings to keep the manufacturing cost low, as is conventionally known (see, e.g., Japanese Laid-open Patent Publication No. 2013-070485).
When a machine such as a machine tool or a robot is controlled in driving by a motor, a motor OFF time is set to be after a predetermined motor ON time to keep the winding temperature below a preset motor temperature protection level (to be referred to as an “alarm level” hereinafter).
Based on, e.g., a track record for actual processing by a machine tool or simulation that uses analysis of a processing program, the motor control device obtains in advance a motor ON time and a motor current value corresponding to one-cycle operation of the machine. The motor control device estimates the winding temperature upon one-cycle operation, based on the obtained motor current value. The motor control device further obtains a motor OFF time to be interposed between one-cycle operations of the machine, to keep the winding temperature below an alarm level upon repetitions of one-cycle operation of the machine. The motor control device prevents burnout of the windings by driving the motor in accordance with a combination (to be referred to as an allowable duty cycle time hereinafter) of the motor ON time and the motor OFF time obtained in advance.
However, since the heat capacity varies in each individual structure forming the machine provided with the motor, it is difficult to estimate the winding temperature in consideration of the heat capacity of the machine and, in turn, to accurately calculate an allowable duty cycle time for the machine.
In such a case, the winding temperature is estimated by trial and error while operating a machine as actually equipped with a motor. Therefore, it takes a lot of time and cost to obtain an allowable duty cycle time for the machine.
Since the motor OFF time is controlled in accordance with the allowable duty cycle time for the machine, it is difficult to efficiently operate the machine when the calculated allowable duty cycle time is inaccurate.